Catalytic systems exist for processing a fuel stream to derive hydrogen, typically for use as a fuel or reactant. One example of such a fuel processing system resides in the fuel processors associated with fuel cell power plants. Fuel cell power plants are becoming well known for a variety of applications requiring a relatively clean and reliable source of electrical power. The power plant typically includes one or more fuel cell stack assemblies (CSA) having anode and cathode regions separated by an electrolyte. The electrolyte may take various forms, with phosphoric acid and solid polymer proton exchange membrane (PEM) being two of the more common, and gives rise to the characterization and operation of the fuel cell.
The generation of electrical power results from an electrochemical reaction occurring between fuel (a reducing agent) supplied at the anode and an oxidant (oxidizing agent) supplied at the cathode. The oxidant is typically air and the reducing agent is hydrogen in a pure, or more typically near-pure, form. The hydrogen may be obtained from various sources, with various forms of hydrocarbon feedstock being the most common. The hydrocarbon feedstock is catalytically processed by a fuel processing system (FPS) also associated with the power plant. The FPS converts the hydrocarbon feedstock to a hydrogen-rich fuel stream via reformation and shift reactions, and may also include selective oxidation to reduce CO levels.
The PEM-type fuel cell is enjoying increased interest, in part because of certain stabilities afforded by the solid membrane electrolyte and in part because of its ability to operate at relatively lower temperatures and pressures and higher current densities. However, the PEM fuel cell also has certain challenging requirements, one of which is the proper management of the water coolant in the system. The coolant water exists as a byproduct of the electrochemical reaction and is used for thermal management in the fuel cell, as a source of water and/or steam for the fuel processing system, and also to maintain appropriate moisture levels at the anode electrode adjacent the PEM membrane.
FIG. 1 illustrates a fuel cell power plant 10 generally in accordance with the prior art. The power plant 10 comprises a fuel cell stack assembly (CSA) 12, a fuel processing system (FPS) 14, a water management/treatment system 16, and may also include a water transfer device (WTD) 18. The CSA 12 is a PEM-type fuel cell, having an anode region 20, a cathode region 22, an electrolyte region 24 between the anode region 20 and the cathode region 22, and a coolant region and flow path 26. The electrolyte is PEM.
A hydrogen-rich fuel stream 28 is supplied to the anode 20 of CSA 12. That fuel stream 28 is derived from a source of hydrocarbon feedstock 30, by means of the FPS 14. The hydrocarbon feedstock may typically be methane, natural gas, gasoline, LPG, naphtha, or the like, and the FPS 14 converts the feedstock, as by reformation, to various component quantities and compounds, including H2, CO, CO2, NH3, etc. As there is normally some sulfur content in the hydrocarbon feedstock 30 which may be deleterious to the catalysts (typically a noble metal such as platinum) in various portions of the FPS 14 and particularly the catalyst of anode 20 of the CSA 12, provision is made for removing, or at least reducing the level of, that contaminant from the fuel stream. That removal is typically accomplished with a desulfurizer, or hydrodesulfurizer (HDS) 32, normally connected between the source of hydrocarbon feedstock 30 and an initial or secondary portion of the FPS 14. The desulfurized feedstock is delivered to the FPS 14 via conduit 33.
The FPS 14 may take a variety of forms, but typically includes at least a hydrogen generator and a shift reactor for segregating the hydrogen from the hydrocarbon feedstock and for shifting resulting CO to CO2. The FPS may also provide subsequent selective oxidation (SOX) to further reduce CO concentrations. While the hydrogen generator may take various forms depending on feedstocks, system dynamics, and/or cost considerations, one common general configuration is that of a partial oxidizer (POX), a catalytic partial oxidizer (CPOX), or an autothermal reformer (ATR), in which the typically desulfurized hydrocarbon feedstock is directly burned or combusted with a supply of oxidant (air) and, except with a POX, water and/or steam in a reformation reaction, which reaction components jointly and severally represent feed streams to the hydrogen generator.
In the illustrated example of FIG. 1, the initial hydrogen generator in the FPS 14 is a CPOX 34, which receives desulfurized hydrocarbon feedstock 30 via HDS 32. An oxidant source apparatus, such as blower 36, supplies the CPOX 34 with an oxidant 38, such as inlet air, passed through the WTD 18. The blower 36, or preferably a separate blower 37, also supplies inlet air as oxidant to the cathode 22 of the CSA 12. The WTD 18 serves to transfer water, otherwise exiting the system, into the incoming air and thereby enhances water balance and energy efficiency of the plant 10. The WTD 18 may be of the general type described in U.S. Pat. No. 6,048,383 to Breault, et al and assigned to the assignee of the present invention, and alternatively referred to as a mass transfer device or, where the transfer of thermal energy is the principal application, an energy recovery device (ERD). The WTD 18 typically includes a mass transfer medium 39, such as one or more plates, membranes, or the like, for permitting mass transfer between the exiting and entering flow streams, while also maintaining their distinct flow paths. The CPOX 34 is also provided with a supply of water and/or steam 40, which may be taken from the coolant flow path 26 and/or the water treatment portion 16 of the system. The CPOX acts in a known manner to catalytically reform, or at least partly reform, the hydrocarbon feedstock 30 in the presence of oxidant 38 and water and/or steam 40. Reference may be made to U.S. Pat. No. 6,299,994 for a better understanding of the relevant reformation and shift reaction formulas, as well as a general functioning of the CPOX 34, with the recognition, however, that the steam and fuel feedstock therein are premixed and reacted in a pre-reforming zone and the resulting effluent is reacted with air in the presence of a catalyst in the POX to provide the reformed effluent stream of H2, CO, CO2 and H2O.
Although only the CPOX 34 of the FPS 14 of FIG. 1 is depicted as an identified block in the FPS, it will be understood that the FPS additionally includes a shift reactor and typically also a selective oxidizer, collectively represented by block 41, with those components functioning in a well-known manner. As noted above, the CPOX 34 and the remaining components of the FPS 14, as well as the catalyst of the anode 20, include catalysts that may be sensitive to various contaminants such as CO and sulfur, and components such as the HDS 32, the shift reactor and the selective oxidizer serve to reduce those contaminants.
The water management/treatment system 16 is provided to maintain coolant water in appropriate amount (supply) and temperature, and also to prevent or minimize problems that might occur as a result of contaminants in the coolant water. The process of reforming hydrocarbon feedstock to produce a hydrogen-rich fuel stream has the normal consequence of introducing various gases, such as NH3 and CO2, into the fuel stream. Those gases tend to dissolve into the water created in the fuel cell and thus enter the coolant. Those dissolved gases in the coolant represent contaminants in that they may cause the conductivity of the water to increase and support destructive shunt current corrosion. Accordingly, the water management/treatment system 16 is provided with an accumulator/degasifier 42 for interaction between an oxidant, typically cathode exhaust air 44 exhausted from the cathode region 22, and coolant water 46 collected in accumulator 42 from the coolant region 26, to facilitate removal of dissolved gases from the coolant.
Coolant water 46 from accumulator/degasifier 42 is circulated to the inlet of coolant flow path 26 in the CSA 12, through the CSA 12 where it acquires gases from the fuel reformation process, and from the CSA 12 for return to the water management/treatment system 16. The coolant exiting from the coolant flow path 26 of the CSA 12 is typically directed through a separator 48 that removes entrained gas bubbles from coolant, then through a radiator 50 for thermal control of the coolant, and is then returned to the accumulator/degasifier 42. Removal of certain dissolved minerals from the coolant water 46 is provided by a demineralizer 52 (DMN) connected in a coolant circuit from the accumulator/degasifier 42 that by-passes the CSA 12 and separator 48 and leads to the radiator 50. A circulation pump 54 connected between the outlets of the separator 48 and the DMN 52 and the inlet to radiator 50 serves to provide the requisite circulation in those liquid circuits. The cathode exhaust air 44 is admitted to the accumulator/degasifier 42 and caused to pass in gas-absorbing contact with the coolant water 46 to cause the gases dissolved in the coolant water to diffuse into the cathode exhaust air 44. That “gas-laden” air, which also contains significant water and some thermal energy, then leaves the accumulator/degasifier 42 via conduit 56 and is conveyed out of the plant via passage through the WTD 18 where water and thermal energy are transferred to the inlet air 38. Further, the exhaust from anode 20 is represented by conduit 58 and may be conveyed directly out of the system, or more typically is utilized for unburned hydrogen content to fuel a burner system (not shown), but in either event may be optionally directed through the WTD 18 (represented by broken line in FIG. 1) with the cathode exhaust 56, in order to recover water and/or thermal energy content it may possess.
An additional understanding of a water management/treatment system may be derived from reference to U.S. Pat. No. 6,207,308 to Grasso, et al and assigned to the assignee of the present invention. That patent describes a water management/treatment system that is similar in many respects to that of FIG. 1, but with the additional provision that it first passes its inlet air, ultimately destined for the cathode, through the degasifying apparatus to cleanse dissolved gases from the water coolant flowing in that degasifying apparatus. Moreover, the inlet air and the water containing the dissolved gases are caused to flow in a counter-current manner in the degasifying apparatus to maximize the release of gas from the coolant water.
While the prior art has addressed concerns with contaminants such as dissolved gases in the coolant water and sulfur in the hydrocarbon feedstock, as discussed with respect to the description of the fuel cell power plant 10 of FIG. 1, an additional concern remains that the level of the sulfur in the fuel stream delivered to the CSA 12 may be excessive as the result of other sources of sulfur. Specifically, the sulfur level target output from the HDS 32 in the fuel stream is 25 ppb/by vol. This fuel then feeds the CPOX 34 (reformer). Assuming no additional sulfur from another source, the 25 ppb becomes about 5 ppb/by vol. coming out of the CPOX 34. The drop in concentration is due to the dilution effect of air and steam that are added in the CPOX process. It has now been recognized that air has about 5 ppb sulfur on average, which contributes to the sulfur coming out of the CPOX. This is particularly true in large urban areas such as the Northeastern United States, where the sulfur level may be 5–10, to as much as 30, ppb/by vol. As a result, the level can be more like 10 ppb or greater, with additional sulfur possibly also coming from the water that is used for the steam. Thus it can be seen that the sulfur in the air can easily double the sulfur load affecting the water gas shift reactor (WGS) catalyst, any possible SOX catalyst, and possibly the catalysts of the fuel cell itself.
Accordingly, it is an object of the invention to provide method and apparatus for improved removal of contaminants from catalytic fuel processors for making hydrogen, and particularly for use in a fuel cell power plant.
It is a further object of the invention to improve the removal of sulfur as a potential contaminant to one or more of the catalysts in the fuel processor and/or cell stack assembly of a fuel cell power plant.
It is yet a further object of the invention to provide a relatively efficient and cost-effective arrangement for the improved removal of contaminants, such as sulfur, in a fuel cell power plant.
It is an even further object of the invention to provide an improved arrangement for reducing the level of sulfur present in the fuel and/or oxidant flow streams of a fuel processor and/or cell stack assembly for a PEM fuel cell power plant.
It is a still further object of the invention to provide a water transfer device (WTD) with enhanced capabilities for the reduction of contaminants.